Conventionally, a magnetic lifting machine as disclosed in Japanese Utility Model Publication No. 1994-55206 comprises a pair of pole plates disposed to be opposite each other on both side ends of the machine and having a non-magnetic medium in the longitudinal center thereof and magnetic members on both sides of the non-magnetic medium; a cylindrical rotor axially supported to be rotatable between the pole plates; a pair of rotational permanent magnets inserted in the rotor and having its magnetic poles opposite to each other; and a pair of fixed permanent magnets disposed on both sides of the rotor. In such a conventional magnetic lifting machine, by rotating the rotor, directions of lines of magnetic force of the rotational permanent magnets with respect to the magnetic members could be changed.
This construction allows the magnetic lifting machine to lift and release ferromagnetic objects depending on on/off position of a switch handle.
For the rotational permanent magnet of the conventional magnetic lifting machine, a ferrite magnet having magnetic flux density under 2,000 gauss was widely used. In order to improve magnetic attractive performance of the lifting machine, however, a relatively large size of the ferrite magnet was required. The large size of the ferrite magnetic necessitated a relatively large size of the magnetic lifting body in the magnetic lifting machine, which was disadvantageous to use and in view of high manufacturing costs as well.
In order to overcome these disadvantages, use of a neodymium magnet having magnetic flux density in the range of 10,000-13,000 gauss has been suggested. However, the neodymium magnet has a fatal disadvantage in that it is easily oxidized when exposed to atmosphere, thereby lowering magnetization.
In order to prevent the above disadvantage, coating on the surface of the neodymium magnet with resin or metal may be suggested. However, use of the coated neodymium magnet in the rotor results in exfoliation of the coated film on the magnet because the neodymium magnet rotates in contact with the pole plates and becomes worn by the friction.
Therefore, in order to solve the above problem, the inventor of the present invention suggested an improved magnetic lifting machine by using the neodymium magnet with excellent coercive force in Korean Patent Publication No. 2002-104765, in which an airtight part kept in vacuum or filled with antioxidant materials is provided in the vicinity of the neodymium magnet to prevent the neodymium magnet from being exposed to or contacted with the air.
This magnetic lifting machine can prevent the oxidation of the neodymium magnet, and thus performance and durability of the machine can be improved. Also, by using the neodymium magnet, the overall volume of the machine can be reduced, enabling users to carry and handle the machine easily.
However, this magnetic lifting machine has disadvantages in that many parts or accessories are required to form the airtight part, thereby increasing manufacturing costs, and the machine becomes complicated.